Method of recovering sulfurous components in a sulfur-recovery process

ABSTRACT

H 2 S is removed from an H 2 S-rich gas, and sulfur is produced, by a process in which the H 2 S-rich gas is reacted with SO 2  in a reactor in the presence of an organic solvent and a catalyst, an H 2 S-containing off-gas is removed from the reactor and is combusted to produce an SO 2 -rich combustion gas. Preferably, the reactor off-gas is combusted with a substoichiometric amount of oxygen so that the combustion gas also contains water vapor and sulfur vapor. The combustion gas is cooled by direct quench or indirect heat exchange to produce an aqueous stream comprising primarily water and containing suspended solid sulfur and polythionic acids, e.g., a Wackenroder&#39;s liquid, and the aqueous stream is used to provide cooling for the H 2 S—SO 2  reaction. Problems associated with production and handling of Wackenroder&#39;s liquids are overcome and sulfur values in these materials are recovered.

CROSS-REFERENCES TO RELATED APPLICATIONS

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REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

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FIELD OF THE INVENTION

The present invention relates to a process of removing hydrogen sulfidefrom natural gas or other industrial gas, in an integrated system wheresulfur is produced.

BACKGROUND OF THE INVENTION

One of the most common systems for processing gases containing hydrogensulfide and producing sulfur involves the use of well-knownabsorber-stripper steps to separate H₂S and the well-known Claus processto produce sulfur. In such system, in simplified form, the basic stepsare usually:

(a) H₂S removal from sour gas, using an H₂S absorbent, to obtainsweetened product gas.

(b) Stripping H₂S out of the H₂S-rich absorbent to obtain H₂S.

(c) H₂S combustion to obtain SO₂ and H₂S.

(d) Solid-catalyzed H₂S reaction with SO₂ at high temperature to formand recover S and to make an off-gas containing reduced amounts of H₂Sand SO₂.

(e) Treating the off-gas from step (d) to recover as S a major fractionof the remaining amounts of H₂S and SO₂ and to form a stack gas that maybe released to the atmosphere.

Steps (c) and (d) in combination are often regarded as the Clausprocess.

A system that is directed to treating sour gas but does not includereaction of H₂S to form sulfur is shown in FIGS. 14-25 of Kohl andRiesenfeld, Gulf Publishing Co., 1985 “Gas Purification”, 3rd Edition.FIGS. 14-25 in the Kohl et al. reference shows the basic steps of (a)H₂S removal from sour gas using an absorbent to take out the H₂S, so asto obtain treated (sweetened gas) of reduced H₂S content out the top ofthe absorber or “contactor” and H₂S-rich absorbent out of the bottom ofthe absorber; and (b) stripping H₂S out of the H₂S-rich absorbent, by aflash regeneration technique and a heated regeneration technique tostrip H₂S from the absorbent and obtain H₂S and regenerated (lean)absorbent for reuse in step (a).

The system illustrated in the Kohl et al. reference uses a physicalabsorbent, such as propylene carbonate.

A chemical solvent could be used in that basic-type system, possiblywithout the flash regeneration part of step (b). Examples of knownchemical-type absorbents include amines, such as monoethanolamine(“MEA”).

Just as Kohl et al. reference at FIGS. 14-25 is directed to H₂Sabsorption/stripping steps, also FIG. 5 from a paper by Lynn et al.,“The University of California Berkeley's Sulfur Recovery Process: ClausRevisited”, 1999 Sulfur Recovery Conference, Austin, Tex., Oct. 24-27,1999, shows the resultant H₂S from absorption/stripping can be routed toa reactor. The reactors illustrated in the October 1999 paper are usedin combination with a Claus plant (see, for example, the furnaceillustrated in FIG. 4). The SO₂-rich gas from the furnace is routed tothe bottom of an SO₂ absorber column. The SO₂ is cooled in the bottom ofthe SO₂ absorber using a cooled organic solvent (SO₂ absorbent) that isrecirculated, through a solvent quench heat exchanger, in a loop at thebottom of the SO₂ absorber.

The October 1999 paper also shows in FIG. 4 a process flow diagram for atypical Shell Claus Off-Gas Treatment (SCOT) unit. It is well known inthe sulfur recovery industry that a SCOT unit may be used downstream ofa Claus plant as a tail-gas clean-up unit (TGCU) to increase therecovery of sulfur from what otherwise would be achieved by only using aconventional Claus plant.

The FIG. 4 illustration of the SCOT unit shows steps including (a)combining a reducing gas with the Claus tail-gas, (b) reducing(hydrogenating) the tail gas containing SO₂, S, COS, and CS₂ in the SCOTreactor to obtain an H₂S-rich stream, (c) quenching the H₂S-rich streamby direct contact with water in a quench tower, (d) H₂Sabsorption/stripping steps to produce an H₂S stream, and (e) recycle ofthe H₂S stream to the Claus plant. Thus, FIG. 4 of the October 1999paper is an example of the use of a direct contact aqueous quench in asulfur recovery process, though to cool an H₂S-rich gas, not an SO₂-richgas, as in the present invention.

Another reference which illustrates a process similar to that shown inFIG. 4 from the October 1999 paper, is Naber et al. “New Shell ProcessTreats Claus Off-Gas”, Chemical Engineering Progress, Vol. 69, No. 12,page 29, December 1973.

The Claus process itself, which consists of a series of reactors inwhich SO₂ and H₂S react to form water and sulfur vapor. The reaction isequilibrium-limited at temperatures above the dew point of sulfur vapor.The gas stream leaving each reactor is near chemical equilibrium. Innormal operation, most of the sulfur is condensed between reactors toallow further reaction in the next stage. However, in the Claus process,the condensers operate above the dew point of water to avoid formingWackenroder's liquid (a dilute aqueous mixture of colloidal sulfur and asolution of sulfoxy acids; see Hackh's Chemical Dictionary, FourthEdition, 1969) and the problems that their formation would present. Thisis done even though the presence of water vapor in the gas stream limitsthe extent of reaction that can be achieved and thus necessitates theinstallation of a tail-gas treatment process.

My prior International patent application WO 99/12849, which is herebyincoporated herein by reference, describes a process in which gaseoushydrogen sulfide (H₂S) reacts with gaseous sulfur dioxide (SO₂) in thepresence of an organic liquid or solvent wherein the following reactionoccurs:

2H₂S(g)+SO₂(g)→3S(l)+2H₂O(g)  (1)

In the reactor, it is desired to operate above the melting point ofsulfur. The reacting gases may flow co-currently or counter-currently toa stream of the organic liquid. A preferred example of such a reactor isa tray-type column in which the reacting gases flow counter-currently toa stream of the organic liquid. The sulfur produced by Reaction (1) ineither type of reactor forms a separate liquid phase that flowsco-currently with the organic liquid.

The gaseous sulfur dioxide is produced by combustion of hydrogensulfide. Preferably this combustion is conducted fuel-rich to avoid therisk of forming SO₃ and NO_(x), both of which are undesirable. However,if the combustion is fuel-rich, then elemental sulfur forms in additionto SO₂ and will be condensed and partially dissolved in the solvent usedin the SO₂ absorber, which is undesirable. On the other hand, if thecombustion is carried out fuel-lean, the free oxygen that accompaniesfuel-lean combustion can cause degradation of the solvent in the SO₂absorber. Furthermore, water vapor is formed by the combustion of H₂Sand any hydrocarbons that are present in the acid gas fed to thefurnace. Most of the water vapor will also condense in the solvent inthe SO₂ absorber. The presence of water vapor together with the SO₂requires additional cooling, or a higher solvent flow, in the absorber.In addition, that water must be boiled out of the solvent in the SO₂stripper, thereby increasing the energy required in operating thestripper. Furthermore, most of this added water vapor must be condensedfrom the SO₂ leaving the stripper before the latter enters the reactorcolumn to avoid an excessive vapor flow within the reactor column.

SUMMARY OF THE INVENTION

According to the present invention, a process is provided for removingH₂S from an H₂S-rich gas and producing sulfur, which comprises:

(a) reacting H₂S in the H₂S-rich gas with SO₂ to produce sulfur and areactor off-gas containing H₂S and H₂O;

(b) combusting the reactor off-gas to produce a combustion gascontaining SO₂, water vapor;

(c) cooling the combustion gas from step (b) to condense water vapor andsulfur and produce an aqueous stream comprising primarily water; and

(d) introducing the aqueous stream from step (c) into the reactor toprovide cooling for the reaction of step (a).

In one aspect, the invention comprises:

in a process for removal of H₂S from an H₂S-rich gas, in which theH₂S-rich gas is reacted with SO₂ in a reactor in the presence of anorganic liquid to produce sulfur, and in which H₂S is combusted toproduce a combustion gas containing SO₂, water vapor, and in which theSO₂ is thereafter reacted with the H₂S-rich gas, the steps comprising:

(a) cooling the combustion gas to condense water vapor and sulfur andproduce an aqueous stream comprising primarily water; and

(b) introducing said aqueous stream into the reactor to provide coolingfor the reaction between the H₂S-rich gas and the SO₂.

In another aspect, the invention comprises:

a process for removing H₂S from an H₂S-rich gas and recovering sulfur,which process comprises feeding the H₂S-rich gas and an SO₂-rich gas,the H₂S being in stoichiometric excess, to a reactor column in thepresence of a solvent that catalyzes their reaction to form liquidsulfur and water vapor; wherein aqueous streams are injected at one ormore points of the reactor column to absorb a part of the heat ofreaction by water vaporization; wherein the H₂S-rich off-gas is scrubbedwith an aqueous stream in the upper section of the reactor column torecover solvent vapor and unreacted SO₂ and is then cooled to condensewater prior to combusting the H₂S-rich off-gas to produce SO₂ to be fedto the reactor column; absorbing SO₂ from the combustion gas bycontacting the gas with an SO₂ absorbent in an absorber to obtain anSO₂-rich absorbent; and stripping SO₂ from the SO₂-rich absorbent toobtain an SO₂-rich gas; which process further comprises:

(a) burning the cooled H₂S-rich gas with an amount of O₂-rich gas in afurnace such that substantially all hydrogen is converted to H₂O, and atleast 90%, preferably about 98% to 99%, of the sulfur is converted toSO₂ while at least 0.1%, preferably about 1% to 2%, of the sulfur isconverted to S vapor;

(b) cooling the SO₂-rich gas from step (a) by direct contact with cooledwater in a separate contacting device or water introduced into the lowerpart of the SO₂ absorber to condense H₂O and S vapor to produce anaqueous slurry containing suspended sulfur and to obtain cooled SO₂-richgas;

(c) absorbing SO₂ from the cooled SO₂-rich gas in an SO₂ absorber bycontacting the gas with an SO₂ absorbent to obtain an SO₂-rich absorbentand a stack gas of low sulfur compound content; and stripping SO₂ fromthe SO₂-rich absorbent to obtain an SO₂-rich gas; and

(d) using the aqueous slurry from step (b) as one of the aqueous streamsinjected at one or more points of the reactor column to absorb a part ofthe heat of reaction by water vaporization.

DESCRIPTION OF THE DRAWING

FIG. 1 is a flow sheet depicting an embodiment of the process of thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a process is provided for removingH₂S from an H₂S-rich gas and producing sulfur, which comprises:

(a) reacting H₂S from the H₂S-rich gas with SO₂ in a reactor column toproduce sulfur and a reactor off-gas containing H₂S;

(b) combusting the reactor off-gas and preferably also any by-passedH₂S-rich gas to obtain an SO₂-rich gas containing water vapor and othergases;

(c) cooling the SO₂-rich gas and water vapor from step (b) to obtain awater stream; and

(d) injecting said water stream into the reactor column to provide apart of the cooling required in the reactor column.

Preferably, the reactor off-gas from step (a) is combusted in step (b)in a high-temperature combustion zone with a substoichiometric amount ofoxygen or air to obtain an SO₂-rich gas that contains a minor amount ofsulfur vapor. By substoichiometric is meant less than the amount ofoxygen that is stoichiometrically required for complete combustion ofthe H₂S, hydrocarbons, organic sulfur compounds, CO and H₂ in thereactor off-gas to form H₂O, SO₂ and CO₂. This feature is particularlyadvantageous in the process of the present invention as thesubstoichiometric amount of oxygen helps avoid, or minimize, theformation of SO₃ in the combusted gas, and thus corrosion concerns areless, both in the cooling step for the SO₂-rich gas and in the reactorcolumn. In addition, the substoichiometric amount of oxygen helps avoidor minimize the formation of NO_(x). This helps achieve a better qualitystack gas exiting the subsequent SO₂ absorption step. Finally, thesubstoichiometric amount of oxygen helps avoid or minimize the amount offree oxygen present in the cooled SO₂-rich combustion gas. This helpsavoid or minimize the degradation of the organic solvent in the SO₂absorber.

It is common practice in furnaces operated with commercial power plantsto follow such a high-temperature, fuel-rich combustion stage with alower-temperature, fuel-lean combustion stage to complete the oxidationprocess. That can be done in the process of the present invention if itis desired to oxidize the sulfur vapor in the combustion gas before itleaves the furnace section. However, in the process of the presentinvention it is preferred to have only a single, fuel-rich combustionoperation because a) the combustion equipment and the combustion controlare simpler, and hence less costly, and b) the substoichiometric amountof oxygen helps avoid or minimize the amount of free oxygen present inthe cooled SO₂-rich combustion gas. This helps avoid or minimize thedegradation of the organic solvent in the SO₂ absorber. However, as isnoted below, such fuel-lean combustion can only be practiced if one hasmeans for advantageously handling the resultant sulfur vapor in thecombustion gas.

By “minor amount of sulfur” is meant relatively low amounts of sulfurcompared to the SO₂ which is formed when the reactor off-gas iscombusted, preferably less than five percent of the total sulfur, andmore preferably less than 2 percent of the total sulfur in the SO₂-richgas. Having a minor amount of sulfur present in the combusted reactoroff-gas helps ensure that a substoichiometric amount of oxygen is used.The oxygen for the combustion most readily will be obtained using air.

In a preferred embodiment of the present invention, the process includesthe following step:

(e) removing SO₂ from the cooled SO₂-rich gas to obtain a stack gas oflow sulfur compound content.

Preferably in the process of the present invention, step (e) is carriedout in an SO₂ absorber wherein SO₂ is removed from the cooled SO₂-richgas by absorption into a solvent in the SO₂ absorber to obtain SO₂-richsolvent.

Also, it is preferred in the present invention that the SO₂ used in step(a) be in gaseous form and obtained at least in part by stripping SO₂from the SO₂-rich solvent. In this embodiment the cooling of theSO₂-rich gas is carried out in the lower part of the SO₂ absorber.

Among other factors, the present invention is based on my concept andfinding that the integrated process of the present invention employs acooling of the wet SO₂-rich gas from an H₂S combustion step, andcondensation of the water and sulfur produced during combustion in sucha way, that it provides an integrated use of the resulting aqueousliquid (i.e. colloidal aqueous suspension or slurry of sulfur) as a partof the coolant in the reactor column (first process step) of the presentinvention. Furthermore, by condensing water from the wet SO₂-richcombustion gas ahead of the SO₂ absorber the absorption of water vaporby the hydrophilic organic solvent in the absorber is minimized orprevented and the need subsequently to strip the water from the solventin the SO₂ stripper is minimized or avoided.

The cooling of the SO₂-rich gas may be carried out with a conventionalpartial condenser, employing indirect heat exchange, to condense waterfrom the wet SO₂-rich combustion gas ahead of the SO₂ absorber. It wouldthen be necessary for the gas/liquid mixture that is formed to flowthrough a gas/liquid separator that included a demister section toseparate water droplets from the gas before the gas stream entered theSO₂ absorber. The gas/liquid separator with demister section could beincorporated in the bottom section of the SO₂ absorber and would thenpreferably be separated from the solvent section by means of a “chimneytray”, that allows passage of the SO₂-rich gas into the solvent sectionwhile preventing flow of solvent into the gas/liquid separation sectionbelow.

Preferably, the cooling of the SO₂-rich gas is accomplished, at least inpart, by direct-contact cooling using a recycling water stream, alsoknown as a water quench. In the process of the present invention, it isparticularly preferred that the portion of recycling water stream thatis used in step (d) is approximately equal to the net water produced incombustion step (a).

In the process of the present invention, preferably the cooling of theSO₂-rich gas is carried out by water introduced to the lower section ofthe SO₂ absorber, and SO₂-rich solvent is withdrawn from an intermediatesection of the SO₂ absorber without having mixed with the water in thelower section of the SO₂ absorber. This is accomplished by having ademister at the top of the quench section plus a chimney tray thatseparates the two sections as described above. Thus, the recycling waterpreferably is a separate circulation loop at the bottom of the SO₂absorber, as is schematically indicated in the drawing. Because of thedemister and the chimney tray the recycling water contains little or nosolvent and the recycling solvent contains little or no elementalsulfur.

It should be noted that in the present process, regardless of whetherthe SO₂-rich gas from the combustion step is cooled by direct contact ina water quench or by indirect heat exchange in a partial condenser, thetemperature to which the SO₂-rich gas from the combustion step is cooledis limited by the temperature of the available cooling medium whether itbe cooling water or ambient air. The SO₂-rich gas can typically becooled to within about 5°-10° C. of the temperature of the availablecoolant; the approach temperature is chosen on the basis of an economicoptimization by methods well known to those skilled in the art. Atypical temperature range for the SO₂-rich gas entering the solventsection of the SO₂ absorber will be 25 to 40° C., with lowertemperatures preferred if not economically prohibitive.

It should also be noted that in the present process, regardless ofwhether the SO₂-rich gas from the combustion step is cooled by directcontact in a water quench or by indirect heat exchange in a partialcondenser, a dilute aqueous solution will be formed that containscolloidal elemental sulfur and sulfurous compounds. This aqueous mixtureof colloidal sulfur and sulfurous compounds is sometimes referred to as“Wackenroder's liquid”. Wackenroder's reaction is the reaction betweenH₂S and SO₂ in aqueous solution to form colloidal sulfur and polythionicacids. (Regarding Wackenroder's reaction, see Hackh 's ChemicalDictionary, 4th Edition, 1969, page 719. Regarding Wackenroder's liquid,see, for example, Yost et al. “Systematic Inorganic Chemistry”, 1946,pages 398 and 399. Yost et al. point out that Wackenroder's liquid iscomplex and contains colloidal sulfur and various sulfurous compounds,including polythionic acids.) Hence, in this embodiment, in addition tothe condensed sulfur at least some of the sulfurous compounds in theaqueous solution are polythionic acids. It should be noted that thevarious sulfurous compounds, including polythionic acids formed inWackenroder's reaction are intermediates in the overall reaction betweenH₂S and SO₂ that form when the reaction occurs in aqueous solution. Inthe organic solvent preferably employed in the present invention theseintermediates are not detected; either they do not form or they reactvery rapidly to complete the reaction to elemental sulfur and water.Furthermore, if a small amount of Wackenroder's liquid is mixed with alarge amount of the preferred solvent, so that the resulting solutiondoes not exceed 10% water by weight, the polythionic acids complete thereaction to elemental sulfur very rapidly and the colloidal sulfurparticles melt and either dissolve in the solvent or form a separateliquid phase.

In other processes it is generally desired to avoid formation ofWackenroder's liquid, and steps are generally taken in prior art sulfurrecovery processes to avoid such formation, as discussed above.

Among other factors, the present invention is based on my concept andfinding that the integrated process of the present invention, in oneembodiment, advantageously embodies use of a direct water quench intoSO₂-rich gas from an H₂S combustion step and provides an integrated useof the resulting aqueous liquid (eg., Wackenroder's liquid) in thereactor column step of the present invention. Further, the resultingWackenroder's liquid contains sulfurous components as well as sulfur,and these sulfurous components are converted to sulfur in the reactorcolumn of the present invention. Still further, the Wackenroder's liquidis preferably used to provide a portion of the cooling in the reactorcolumn.

Preferably in the present invention, the amount of colloidal sulfur andsulfurous compounds is at least 0.5 percent by weight (calculated assulfur) of the total aqueous liquid. More preferably the amount ofcolloidal sulfur and sulfurous compounds in the Wackenroder's liquidformed in the process of the present invention is at least one percent(calculated as sulfur), most preferably 2-4 percent (calculated assulfur) of the total aqueous liquid.

According to a preferred embodiment of the present invention, SO₂ isremoved from the SO₂-rich gas using a lean solvent introduced to anupper part of the SO₂ absorber and SO₂-rich solvent is removed from anintermediate part of the SO₂ absorber. Preferably in the process of thepresent invention, the SO₂-rich solvent is stripped of SO₂ in an SO₂stripper, and resulting SO₂ is fed to step (a) for reaction with theH₂S.

According to a preferred embodiment of the present invention, a secondaqueous stream (a different stream from the Wackenroder's liquiddescribed above) is injected in one or more points of the upper part ofthe reactor column (for example, as schematically illustrated in thedrawing) to provide cooling in the reactor column, and to assist insolvent vapor recovery and unreacted SO₂ recovery.

In a sulfur recovery process of the type described in my prior patentapplication, WO 99/12849, which is one type of process in which thepresent invention may be employed, gaseous hydrogen sulfide (H₂S) reactswith gaseous sulfur dioxide (SO₂) in the presence of an organic liquidwherein the following reaction occurs:

2H₂S(g)+SO₂(g)→3S(l)+2H₂O(g)  (1)

The source of the H₂S-rich gas for the present invention may be aconventional absorber/stripper operation that removes H₂S from a sourindustrial gas, or may be an industrial operation that produces such agas directly. In general, the higher the concentration of H₂S in theH₂S-rich gas the better, but gases containing 20 vol % H₂S or even lowercan be processed by the method of this invention.

The solvent, also of the type described in my International Application,which patent application is incorporated herein by reference, ispreferably miscible with water, has a low volatility, is a relativelygood solvent for both reactants, catalyzes the reaction and is one inwhich liquid sulfur has a limited but low solubility. By low volatilityis meant a normal boiling point of 150° C. or higher, preferably 180° C.or higher. A relatively good solvent is one in which the solubilities ofboth reactants exceed their respective solubilities in water by at leasta factor of ten at 120° C. Low sulfur solubility is less than 10 percentweight sulfur, preferably lower, in the solvent at 120° C. Preferredsolvents for the reactor column include polyethyleneglycol ethers, suchas the methyl ether of triethylene glycol, the dimethyl ether oftriethylene glycol, and the dimethyl ether of polyethylene glycol. Themethyl ether of diethyleneglycol is particularly preferred for use inthe reactor column in the process of the present invention. The solventused in the reactor column may also be the catalyst for the reaction ofH₂S with SO₂ to form sulfur. However, according to a preferredembodiment of the present invention, a catalyst is added to the solventto catalyze or enhance catalysis of the reaction of H₂S with SO₂ to formsulfur. Preferred catalysts include N-substituted aromatic-ringcompounds in which there is no moiety attached to a carbon adjacent to aring nitrogen, such as pyridine, isoquinoline or 3-methyl pyridine. Thecompound 3-hydroxymethyl pyridine is a particularly preferred catalystfor use in the reactor column in the present invention.

In the reactor, one example of which is a packed column similar to thoseused in gas absorption, it is desired to operate above the melting pointof sulfur. The reacting gases flow co-currently to a stream of theorganic liquid. A preferred example of a reactor is a tray-type columnin which the reacting gases may flow co-currently or counter-currentlyto a stream of the organic liquid. The sulfur produced in either type ofreactor forms a separate liquid phase that flows co-currently with theorganic liquid.

In the present invention, the furnace is preferably operated slightlyfuel-rich and a quench operation (i.e., a partial condenser) providesfor condensing a large fraction of the water in the combustion gas whileavoiding the problem of sulfur collecting in the absorber solvent. Thequench operation may employ either a conventional heat exchanger or adirect-contact stream of water. Further, the present invention utilizescondensate formed by cooling the combustion gas as cooling water in thereactor column.

An embodiment of the process of the present invention is shown in FIG.1. At the bottom of the SO₂ absorber, a quench section is schematicallyshown. Preferably water is circulated in the quench section. At thebottom of the quench section, the water is pumped through a suitableheat exchanger, cooled, and most is returned to the top of the quenchsection. The net amount of water formed by combustion of the acid gasmust be removed from the quench-circulation system. The furnace orcombustion step preferably is operated with an O₂/fuel ratio that isabout equal to the stoichiometric requirement to do the following: a)convert at least 95%, preferably about 98% to 99% of the H₂S to SO₂+H₂O;b) convert at least 0.1%, preferably 5% or less, most preferably about1% to 2% of the H₂S to S₂+H₂O; and c) convert substantially all of theorganic sulfur compounds to SO₂+CO₂+H₂O and substantially all of thehydrocarbons and other combustibles to CO₂+H₂O. The SO₂ content of thecombustion gas in the process of the present invention preferably issufficiently high to prevent the formation of any substantial amount ofsoot, CO, COS, CS₂ or H₂ whereas the S₂ content is sufficiently high toprevent the formation of any substantial amount of SO₃ or NO_(x).(Although the term “substantial” is a relative term, in the foregoingcontext and elsewhere in the present specification, the term“substantial” denotes less than 0.5 percent by weight, preferably lessthan 0.1 percent, and most preferably less than 100 ppm.) Thecomposition of this combustion gas is markedly more oxidizing in naturethan is the combustion gas in a conventional Claus furnace, in whichonly about one-third of the H₂S has been converted to SO₂. In the lattercase, because the gas is much more reducing in nature, the formation ofsoot, CO, COS, CS₂ and H₂ are much more strongly favored.

With a typical acid gas, the ratio of S to water is about 2% to 4% byweight when the combustion gas is quenched. Such a slurry is easilypumped since the sulfur particles are of colloidal dimensions and haveno tendency to stick to piping or the vessel walls. Indeed, it ispossible for the fraction of H₂S burned to form sulfur vapor to bevaried by a factor of 2 or even more without causing mechanical orchemical problems with the operation of the quench system in the processof the present invention. At the temperature of the quench, elementalsulfur is essentially non-volatile so that it can be completely removedfrom the combustion gas. The quantity of water condensed from thecombustion gas forms roughly one-third of the amount required to providethe cooling in the reactor column described above.

As the combustion gas flows through the waste-heat boiler following thefurnace, a small amount of the sulfur will be converted back to H₂S bythe reverse of Reaction (1) in the gas phase. In the quench section,this H₂S will react with SO₂ and water to form a very dilute solution ofsulfoxy acids, sometimes referred to as sulfonic acids, which, combinedwith colloidal sulfur, is known in the technical literature as“Wackenroder's liquid”. This mixture would normally present a difficultdisposal problem. However, in the process of this invention, the quenchliquid is injected into the reactor column, usually into the solventstream flowing through the reactor column, as part of the coolant waterto absorb heat from Reaction (1). The volume of this coolant is smallcompared to the volume of solvent flowing through the reactor column,and the solvent is miscible with water. Upon injection, the colloidalsulfur mixes with and is dissolved by the solvent; the sulfoxy acids mixwith the solvent and become part of the reacting system within thecolumn, and the water evaporates as noted above.

It should be noted that, in general, the use of a partial condenser tocool a gas stream is not novel. However, in a preferred embodiment thepresent invention involves a combination of a partial condenser—in whichwater and sulfur vapors condense while H₂S and SO₂ react—with aparticular utilization of the resultant aqueous mixture in an integratedprocess that includes injection of the resultant aqueous mixture intothe reactor column of the present invention so that there is no netaqueous effluent from the quench operation.

The reactor used in the process of the present invention is preferably areactor column. The term “column” is used to denote that the reactorvessel is preferably a column of the type used in fractionaldistillation or gas absorption. Fractional distillation and gasabsorption are well-known arts, and the basic form of such a column iswell known: elongated vessels with trays or packing or even “bales” ofmaterial. The trays can be weep hole trays or bubble cap trays.Regardless of the internals, in the preferred embodiment the basicconcept is to have countercurrent flow, with liquids traveling downwardand vapors upward. Absorption occurs in an absorption column as certaincomponents in the entering gas mixture are absorbed by a solventdescending from overhead. In the preferred reactor column of the presentinvention, liquid solvent flows downward and gases including H₂S and SO₂flow upward. Parts of the H₂S and SO₂ are dissolved in the organicsolvent and the H₂S reacts with the SO₂ in the liquid phase to formsulfur and water vapor. As a stoichiometric excess of H₂S is usedrelative to SO₂, the gas exiting the reactor column still containsunreacted H₂S, and the gas is referred to as an H₂S-rich off-gas.

The term “H₂S-rich off-gas” is used herein not only for the gas exitingthe top of the reactor column, but also is used to follow that gasstream through the reactor column overhead system and into the furnace.The overhead system preferably will include, in the process of thepresent invention, a cooling step, to generate “reflux” condensate forthe reactor column (reflux being another aspect common to gas absorptionand fractional distillation).

Referring again to the reaction of SO₂ with H₂S in the reactor column,at least part of the SO₂ preferably enters the column a tray or twobelow the entry of the H₂S-containing stream so that the liquid sulfuris stripped of H₂S before it leaves the column. At the bottom of thecolumn, the two liquids are separated by decantation, the organic liquidis recycled to the top of the column whereas the liquid sulfur forms aproduct of the process. The walls of the reactor column and of thepiping through which liquid flows preferably are heated as necessary tomaintain a temperature in the range 120° C.-150° C., preferably between125° C.-140° C., to prevent deposit of solid sulfur.

The temperature inside the reactor preferably is maintained in the range120° C.-150° C., more preferably in the range 125° C.-140° C.Preferably, the temperature is maintained by injecting water at one ormore points in the column. The evaporation of the water absorbs most ofthe heat of the reaction; the energy released by Reaction (1) is about3.4 times the molar heat of vaporization of H₂O. Preferably, a heatexchanger is used in the solvent pump-around line to remove part of theheat of reaction during operation, as well as to heat the system priorto startup. The reactor column preferably operates at a pressurenominally equal to that of the H₂S stripper, of the order of 1.5 to 3atmospheres absolute. However, the reaction pressure is not limited tothat range but could be as high as 5 atmospheres when using conventionalequipment. The higher the pressure, the more rapid will be the reactionbetween the two gases.

In the process of the present invention, preferably H₂S is in excessrelative to SO₂ at all points above the entry of the H₂S-rich gas to thereactor column, and the unreacted H₂S, together with any co-absorbedcomponents that are inert in the reaction, leaves the column, preferablyafter passing through a scrubbing section to recover solvent vapor andpreferably after a cooling section to condense water vapor. Any SO₂ inthe gas leaving the solvent section of the reactor column reacts veryrapidly with H₂S in the water-scrubbing section via Reaction (1),forming a very dilute Wackenroder's liquid (described above). Theaqueous mixture leaving the scrubbing section is injected back into thereactor column as part of the coolant. The condensate is sent to asour-water stripper where it is freed of dissolved H₂S and becomes aproduct of the process.

FIG. 1 is a simplified process-flow diagram that shows the majorcomponents and general operating conditions of one embodiment of theprocess of the invention. FIG. 1 illustrates the use of a reactor columnemploying counter-current flow of the gases and liquids, with the liquidstreams flowing down and the gases flowing up. The counter-currentcolumn can employ packing or more preferably trays such as are used ingas absorption columns. To facilitate the description, items ofequipment are given three-digit numbers whereas streams are given one-or two-digit numbers. A given stream maintains the same number as itflows through pumps and heat exchangers as long as its composition isunchanged. In the description below the numbers referring to streams arebetween parentheses whereas the numbers referring to items of equipmentare without parentheses.

In FIG. 1 a stream of H₂S-rich gas, (1), that is typical of a streamrecovered from a sour industrial gas by an absorber/stripper operation(not shown), enters the system. A major fraction of stream (1) is sentvia stream (2) to Reactor Column 101. A minor fraction of stream (1)bypasses Reactor Column 101 via stream (6). Stream (6) is preferably 30%or less of stream (1), is more preferably 20% or less of stream (1), andis most preferably 15% or less of stream (1). The SO₂-rich stream fed toReactor Column 101 is stream (24). The source of stream (24) is SO₂Stripper 100, the operation of which is discussed below. Preferably atleast part of the SO₂-rich stream (24A) enters Reactor Column 101 at thesame level as H₂S-rich stream (1) whereas a smaller part of the SO₂-richstream (24B) enters Reactor Column 101 one or two trays below H₂S-richstream (1) and serves to strip and react away dissolved H₂S from thedescending sulfur and solvent streams before they exit from ReactorColumn 101. The solvent stream fed to Reactor Column 101 is stream (36),which has been decanted from sulfur stream (60) at the bottom of ReactorColumn 101. By regulating the fraction of the part of the SO₂-richstream flowing in stream (24B) only minor amounts of SO₂ are present insolvent stream (36) and sulfur stream (60). During normal operationsolvent stream (36) is pumped by Pump 120 through Heat Exchanger 260 andis cooled to a temperature of about 120° C. During start-up of thesystem, steam is supplied to Heat Exchanger 260 to preheat the solventbefore the start of operations.

Solvent stream (36) enters near the top of Reactor Column 101, below thewaterwash section described below. The two gaseous reactant streams, (2)and (24), combine near the bottom of Reactor Column 101 and thereactants are absorbed by and react in the solvent phase to form watervapor and a second liquid phase of elemental sulfur. In addition,coolant water is injected into the solvent at various points alongReactor Column 101 and in turn vaporizes from the solvent so that thedesired range of temperatures is maintained. At the bottom of ReactorColumn 101 the liquid streams flow into a liquid/liquid separatorsection. Liquid sulfur settles rapidly to the bottom of the section. Thetwo liquids are decanted; the sulfur is one of the products of theprocess, stream (60), and is removed via pump (110) whereas the solventstream (36) flows to Pump 120 as noted above.

In the top section of Reactor Column 101 gas stream (5) is scrubbed withaqueous stream (21) to remove solvent vapor, react away residual SO₂ andprovide coolant as noted above. H₂S and SO₂ react very rapidly in waterto form colloidal sulfur and various sulfoxy acids as noted above sothat gas stream (5) is free of both solvent vapor and residual SO₂ whenit enters Heat Exchanger 240. Most of the water is condensed from stream(5) and is separated in a sour aqueous stream (22) that is split intostreams (20) and (21). Dissolved H₂S is stripped from stream (20) inSour Water Stripper 102 and stream (20) becomes the pure water productfrom the process. The off-gas from Stripper 102, a very'small stream(SA), is combined with the reactor off-gas, stream (5), which flows toHeat Exchanger 240. Scrubbing water, stream (21), is separated fromstream (22) and fed to the scrubbing section at the top of ReactorColumn 101 as noted above. The scrubbing liquor leaving the scrubbingsection is conveyed via pump (115) and line 21A and injected into thesolvent at various points along Reactor Column 101 as noted above.

The cooled H₂S-rich gas, stream 5, from Heat Exchanger 240 combines withReactor By-Pass Gas stream (6) to become stream (7), the H₂S-rich feedto -Furnace 104. The air flow to Furnace 104, stream (8, 9), is providedby Blower 140. The furnace preferably is operated with an O₂/fuel ratiothat is about equal to the stoichiometric requirement for convertingabout 98% to 99% of the H₂S to SO₂+H₂O, preferably about 1% to 2% of theH₂S to S₂+H₂O, substantially all of the organic sulfur compounds toSO₂+CO₂+H₂O and substantially all of the hydrocarbons and othercombustibles to CO₂+H₂O. The SO₂ content of the combustion gas in theprocess of the present invention preferably is sufficiently high toprevent the formation of any substantial amount of soot, CO, COS, CS₂ orH₂ whereas the S₂ content is sufficiently high to prevent the formationof any substantial amount of SO₃ or NO_(x).

As the combustion gas flows through the waste-heat boiler followingFurnace 104 and is cooled, a small amount of the sulfur is convertedback to H₂S by the reverse of Reaction (1) in the gas phase. Gas stream(10), which is at a temperature of about 150° C., then enters the quenchsection at the bottom of SO₂ Absorber 103. In the quench section stream(10) is cooled to near ambient temperature and the sulfur vapor and mostof the water vapor formed by the combustion are condensed by quenchstream (31A, 234). The heat absorbed by quench stream (31A) is removedin Heat Exchanger 230. The H₂S in stream (10) will react with SO₂ andwater in the quench operation to form a very dilute solution of sulfoxyacids, which, combined with colloidal sulfur, is known in the technicalliterature as “Wackenroder's liquid”. The net material condensed in thequench operation leaves the quench section of SO₂ Absorber 103 in stream(31) arid becomes a part of the coolant supplied to Reactor Column 101.The cooled SO₂-rich gas flows counter-currently to the cooled solventstream (35) in SO₂ Absorber 103 and the SO₂ is absorbed to form SO₂-richsolvent stream (32). The relative quantities of cooled solvent stream(35, 30), cooled SO₂-rich stream (10) and the height of SO₂ Absorber 103are such that the stack gas, stream (11), leaving SO₂ Absorber 103 meetsambient air-quality standards, typically 100 parts per million SO₂ orless. A small water stream (24) washes solvent vapor from the stack gasat the top of SO₂ absorber 103. Stream (32) is p heated in HeatExchanger 210 by lean solvent stream (34) and flows to SO₂ Stripper 100.

In SO₂ Stripper 100 the rich solvent flows counter-currently to thestripping vapor generated in Reboiler 200. A major fraction of thatvapor consists of water that has been boiled from the only-moderatelyvolatile solvent, and that was added as reflux stream (23) at the top ofthe column. Reflux stream (23) also serves the purpose of scrubbingsolvent vapor from the SO₂-rich gas stream (24) leaving the top of SO₂Stripper 100. SO₂-rich gas stream (24) then passes through Condenser205, where a major fraction of the water content condenses to formstream (23). SO₂-rich solvent stream (32) typically contains somewhatmore water than lean solvent stream (34); this excess water is sent toReactor Column 101 in stream (23A) to act as coolant. Hot, lean solvent,stream (34), leaves the bottom of SO₂ Stripper 100 and is pumped by Pump105 back to SO₂ Absorber 103 by way of Heat Exchangers 210 and 220,being cooled in the process.

EXAMPLE

Referring to FIG. 1, stream (1) has an hourly flow of 222 kmol H₂S, 107kmol CO₂, 0.7 kmol CH₄ and 0.3 kmol H₂O. The daily production of liquidsulfur is 170 tonnes (1 tonne=1000 kg). Stream (2), the H₂S-rich feed toReactor Column 101, constitutes the entire flow of stream (1). Stream(24) feeds 73.8 kmol/hr SO₂ to Reactor Column 101. Reactor Column 101consists of 12 theoretical stages resembling those in a bubble-cap gasabsorption column. The solvent flow through Reactor Column 101, stream(36), is 350 kmol/hr. The solvent is diethylene glycol methyl ether. Onthe feed tray for the H₂S-rich feed to Reactor Column 101 about 54% ofthe SO₂ reacts. On the top stage about 0.6% of the SO₂ reacts, for atotal of 99.7%, and 74.4 kmol/hr H₂S remain in the reactor off-gas,stream (5). The total reactor coolant, streams (31)+(22), contains 155kmol/hr H₂O, 31 kmol/hr solvent and 1.4 kmol/hr S. Condenser 250separates about 350 kmol/hr H₂O from the reactor off-gas; about 150kmol/hr is fed via stream (21) to the scrub section at the top ofReactor Column 101 and about 200 kmol/hr passes through Sour WaterStripper 102 and becomes a product of the process, stream (20).

The air flow to Furnace 104, stream (9), contains 424.7 kmol/hr N₂,112.5 kmol/hr O₂ and 9.4 kmol/hr H₂O (assuming 50% relative humidity).The adiabatic flame temperature is about 1400° C. (2560° F.). Thequantity of sulfur vapor is 0.75 kmol/hr; there is no CO, COS, CS₂ or H₂in the combustion gas. The combustion gas, stream (10), also contains73.8 kmol/hr SO₂, 108 kmol/hr CO₂ and 95.4 kmol/hr H₂O in addition tothe nitrogen in the air.

The quench stream (31A) contains 2500 kmol/hr H₂O and 31 kmol/hr S; thenet quench liquid, stream (31), contains 61 kmol/hr H₂O and 0.75 kmol/hrS. The flow of solvent through SO₂ Absorber 103 is 750 kmol/hr; SO₂Absorber 103 has 6 theoretical stages for the quench section and 12theoretical stages for the solvent section. The lean solvent, stream 35,is diethylene glycol methyl ether and contains about 4 wt % water. Thestack gas, stream 11, from SO₂ Absorber 103 contains 0.044 kmol/hr (80ppmv) SO₂.

SO₂ Stripper 100 has 12 theoretical stages for the stripping section and2 stages above the solvent feed stage for the reflux section. The vaporflow from the reboiler is 230 kmol/hr H₂O and 20 kmol/hr solvent. Theamount of SO₂ in the lean solvent is 0.0022 kmol/hr, 0.003% of thatentering in stream 33. Reflux stream 23 is 93 kmol/hr H₂O.

What is claimed is:
 1. A process for removing H₂S from an H₂S-rich gasand producing sulfur, which comprises: (a) reacting H₂S from theH₂S-rich gas with SO₂ in a reactor to produce sulfur and a reactoroff-gas containing H₂S and H₂O; (b) combusting the reactor off-gas toproduce a combustion gas containing SO₂, water vapor, and sulfur vapor;(c) cooling the combustion gas from step (b) to condense water vapor andsulfur vapor and produce an aqueous stream containing sulfur; and (d)introducing the aqueous stream from step (c) into the reactor to providecooling for the reaction of step (a).
 2. A process in accordance withclaim 1 wherein the aqueous stream from step (c) comprises primarilywater.
 3. A process in accordance with claim 1 wherein the cooling ofthe combustion gas is performed with a direct water quench.
 4. A processin accordance with claim 1 wherein the cooling of the combustion gas isperformed by indirect heat exchange with a cooling medium.
 5. A processin accordance with claim 1 which further comprises: (e) removing SO₂from the cooled combustion gas to obtain a stack gas containing 100 ppmor less of SO₂.
 6. A process in accordance with claim 5 wherein step (e)is carried out in an SO₂ absorber wherein SO₂ is removed from the cooledcombustion gas by absorption into a solvent to obtain SO₂-rich solvent.7. A process in accordance with claim 6 wherein SO₂ used in step (a) isobtained at least in part by stripping SO₂ from the SO₂-rich solvent. 8.A process in accordance with claim 7 wherein the cooling of step (c) iscarried out in the lower part of the SO₂ absorber.
 9. A process inaccordance with claim 8 wherein the cooling of step (c) is performedwith a direct water quench.
 10. A process in accordance with claim 6wherein SO₂ is removed from the combustion gas using an SO₂-lean solventintroduced into an upper part of the SO₂ absorber and wherein SO₂-richsolvent is removed from an intermediate part of the SO₂ absorber.
 11. Aprocess in accordance with claim 1 wherein step (a) is conducted in thepresence of a solvent; the reactor off-gas, before leaving the reactor,is contacted with a second aqueous stream to recover solvent vapor,unreacted SO₂ reacts with H₂S in the reactor off-gas in the presence ofthe water contained in the second aqueous stream to produce a thirdaqueous stream comprising primarily water and containing suspendedsulfur, and wherein the third aqueous stream is introduced into thereactor to provide cooling for the reaction of step (a).
 12. A processin accordance with claim 1 wherein the amount of sulfur in the aqueousstream from step c) is from about 0.1 to about 10 percent by weight. 13.In a process for removal of H₂S from an H₂S-rich gas, in which theH₂S-rich gas is reacted with SO₂ in a reactor in the presence of anorganic liquid to produce sulfur, and in which H₂S is combusted toproduce a combustion gas containing SO₂, water vapor and gaseous sulfur,and in which the SO₂ is thereafter reacted with the H₂S-rich gas, thesteps comprising: (a) cooling the combustion gas to condense water andsulfur vapor and produce an aqueous stream comprising primarily waterand containing suspended sulfur; and (b) introducing said aqueous streaminto the reactor to provide cooling for the reaction between theH₂S-rich gas and the SO₂.
 14. A process in accordance with claim 13wherein the cooling of the combustion gas is performed with a directwater quench.
 15. A process in accordance with claim 13 wherein thecooling of the combustion gas is performed by indirect heat exchangewith a cooling medium.
 16. A process in accordance with claim 13 whichalso comprises: (c) removing SO₂ from the cooled combustion gas toobtain a stack gas containing 100 ppm or less SO₂.
 17. A process inaccordance with claim 16 wherein step (c) is carried out in an SO₂absorber having an upper portion and a lower portion, wherein SO₂ isremoved from the cooled combustion gas by absorption into a solvent toobtain SO₂-rich solvent.
 18. A process in accordance with claim 17wherein SO₂ that is reacted with the H₂S-rich gas is obtained at leastin part by stripping SO₂ from the SO₂-rich solvent.
 19. A process inaccordance with claim 17 wherein the cooling of the combustion gas iscarried out in the lower portion of the SO₂ absorber.
 20. A process inaccordance with claim 19 wherein the cooling of the combustion gas isperformed with a direct water quench.
 21. A process in accordance withclaim 17 wherein SO₂ is removed from the combustion gas using anSO₂-lean solvent introduced into an upper part of the SO₂ absorber andwherein SO₂-rich solvent is removed from an intermediate part of the SO₂absorber.
 22. A process in accordance with claim 13 wherein the reactionbetween the H₂S-rich gas and the SO₂ is conducted in the presence of asolvent to produce a reactor off-gas; the reactor off-gas, beforeleaving the reactor, is contacted with a second aqueous stream torecover solvent vapor, unreacted SO₂ reacts with H₂S in the reactoroff-gas in the presence of the water contained in the second aqueousstream to produce a third aqueous stream comprising primarily water andcontaining suspended sulfur, and wherein the third aqueous stream isintroduced into the reactor to provide cooling for the reaction of theH₂S-rich gas with the SO₂.
 23. A process in accordance with claim 13wherein the amount of sulfur in the aqueous stream is from about 0.1 toabout 10 percent by weight.
 24. A process for removing H₂S from anH₂S-rich gas and producing sulfur, which comprises feeding the H₂S-richgas and an SO₂-rich gas, the H₂S being in stoichiometric excess, into areactor column in the presence of a solvent that catalyzes theirreaction to form liquid sulfur and water vapor; wherein a first aqueousstream is injected at one or more points of the reactor column to absorba part of the, heat of reaction by water vaporization; wherein theH₂S-rich off-gas is scrubbed with a second aqueous stream in the uppersection of the reactor column to recover solvent vapor and unreacted SO₂and is then cooled to condense water; combusting the H₂S-rich off-gas toproduce SO₂ to be fed to the reactor column; absorbing SO₂ from thecombustion gas by contacting the gas with an SO₂ absorbent in anabsorber to obtain an SO₂-rich absorbent; and stripping SO₂ from theSO₂-rich absorbent to obtain an SO₂-rich gas; which process furthercomprises: (a) burning the cooled H₂S-rich off-gas with an amount ofO₂-rich gas in a furnace such that, substantially all hydrogen isconverted to H₂O, and at least 90% of the sulfur is converted to SO₂while at least 0.1% of sulfur is converted to sulfur vapor; (b) coolingthe SO₂-rich gas from step (a) by direct contact with cooled water toproduce an aqueous slurry containing solid sulfur; (c) absorbing SO₂from the cooled SO₂-rich gas in an SO₂ absorber by contacting the gaswith an SO₂ absorbent to obtain an SO₂-rich absorbent; (d) stripping SO₂from the SO₂-rich absorbent to obtain an SO₂-rich gas; and (e) using theslurry of solid sulfur suspended in water from step (b) as a part of thefirst aqueous stream injected at one or more points of the reactorcolumn to absorb a part of the heat of reaction by vaporization.
 25. Aprocess for removing H₂S from an H₂S-rich gas and producing sulfur,which comprises: (a) reacting H₂S from the H₂S-rich gas with SO₂ in areactor in the presence of an organic liquid to produce sulfur and areactor off-gas containing H₂S and H₂O, wherein the SO₂ is introducedinto the reactor as a gas; (b) combusting the reactor off-gas to producea combustion gas containing SO₂ and water vapor; (c) cooling thecombustion gas from step (b) to condense water vapor and produce anaqueous stream; (d) recovering SO₂ from the cooled combustion gas; (e)introducing SO₂ from step (d) into the reactor as the SO₂ gas of step(a); and (f) introducing the aqueous stream from step (c) into thereactor to provide cooling for the reaction of step (a).